it is not possible due to 1 simple reason, stamina cost to initiate run - 25, stamina cost to cancel fireball - 25.
Basicly when you initiate FBRC, you lose 50 stamina, if you do next one perfectly or not - it will not cancel by running because there is stamina check before you cancel your fireball, and if you don't have 75 stamina or more, it would'nt allow you to cancel into RUN because you will not have 1 full bar of stamina to initiate the run, and instead you will get a DASH.
You can runcancel twice only in 1 scenario, where first run cancel will be on block, and second one will be on hit after some time when your stamina will recover. Doing 214 FBRC 214 is NOT run cancelling twice. Pre patch you could do 214 FBRC 214 FBRC 214
And before the patch, initiating run and cancelling your special was 15/15 stamina instead of 25/25 right now, so doing 2 run cancels only required you to not fuck up 5 frame window (from 70 to 65 stamina), because you would need 15 stamina to cancel your special before it touches your second bar of stamina - so you can initiate the run.